1. Field of the Invention
The present invention relates to an input circuit for an analog input module used to process an analog input signal from a process control sensor to provide an input signal for a Programmable Logic Controller (PLC).
2. Discussion of the Background
Methods of controlling industrial processes have evolved from the beginning of industrial manufacture at the end of the nineteenth century up to the present day. To begin with, each step of a process had to be carried out by a human operator, with or without the aid of a machine. Later, systems for automatic control evolved, at first using purely mechanical means, and later using electrical control systems. With the advent of digital logic, and then computers, analog electrical control systems have largely given way to digital systems. Modern process control systems are typically controlled by a programmable logic controller (PLC). A PLC is essentially a computer containing a Central Processing Unit (CPU) and memory. A PLC may be programmed typically from a terminal or from a general purpose computer. A PLC also has inputs provided for signals derived from sensors and outputs provided to control actuators. The sensors may measure process control variables such as pressure, temperature, flow rates, pH, oxygen content or any number of other variables from which proper operation of a process may depend. The actuators may, for example, control valves, solenoids or the like. Process control systems employing a PLC may be used to control any type of process in automotive plants, printing plants, breweries, or any other type of industry, and lower cost systems may be employed for home automation. Although the PLC is a digital device, the sensors and actuators employed in the process control system are often analog devices. For example, a pressure or temperature sensor may produce a voltage or current which varies continuously, i.e., in an analog fashion, dependent upon the quantity measured. Similarly, an actuator may, for example, control the position of a valve to regulate a flow dependent upon a value of an analog voltage. Analog input and output modules, collectively input/output (I/O) modules, are normally employed to convert the voltages and currents received from the sensors into digital values that can be processed by the PLC, and to convert the digital output values from the PLC into voltages or currents that can be used to control actuators.
An analog input module for a PLC may include an input circuit to convert an input analog voltage or current range to a voltage range suitable for an analog to digital converter, followed by an analog to digital converter, an optical isolator for safety reasons, and then a microcontroller. The microcontroller or any other suitable device provides an input to the PLC. It will be understood by those skilled in the art that an analog input module may be used with a controller other than a PLC, such as a vision controller, an instrument controller, a data acquisition system, motion controller, or any computer. Examples of analog input modules are the Quantum, Kampai and 984 modules produced by Schneider Automation.
A difficulty arises in that several different voltage and/or current ranges may be provided by the different types of analog input sensors. One solution to this problem is to provide different types of analog input modules, each having a different voltage or current input range. A disadvantage with that approach, however, is that a number of different types of modules need to be provided and each one can only be used with certain types of sensors, so that the sensor has to be matched up with the analog input module having the same voltage or current range.
Another solution to this problem is to provide a multi-range analog input module. Such a module has up until now needed a switching mechanism to switch between different input circuits, depending on the voltage or current range to be input to the module. This is further complicated by the fact that it is desirable to control such switching remotely, i.e., from the PLC. The input modules are normally located close to the sensors, which may not be close to the PLC, and which in fact may be in hazardous or otherwise inaccessible locations. This is particularly but not exclusively true in the control of chemical processes, where toxic and/or explosive materials are processed. Consequently, it is desirable for such switching arrangements to be controlled electronically rather than mechanically, which greatly increases the component count in the input circuits and their attendant costs.
The present invention provides a novel input circuit. According to one embodiment, the input circuit forms part of an analog input module for a programmable logic controller or other control device. The analog input module includes an analog-to-digital converter and an input circuit with an output terminal connected to the input of the analog-to-digital converter, a voltage input terminal, a current input terminal, and a common terminal. The input circuit is configured to accept a voltage input between the voltage input terminal and the common terminal, to accept a current input between the current input terminal and the common terminal, and to provide an output voltage at the output terminal dependent upon either the voltage input or the current input, without switching between them.
A preferred embodiment of an analog input module according to the invention will be further described with reference to the drawings.